Transistor-based molecular detection apparatus and method

ABSTRACT

Adjacent first and second transistors are integrated with a substrate. Each of the first and second transistors has a gate electrode, a source electrode, a drain electrode and a semiconductive channel formed of an organic material, the semiconductive channel electrically coupling the source electrode with the drain electrode. The source electrode of the first transistor is electrically coupled to the source electrode of the second transistor. A molecular receptor is bound directly to a surface of the semiconductive channel of the first transistor. A non-zero offset voltage, which produces equal channel currents in the semiconductive channels of the first and second transistors after a molecule has bound with the molecular receptor without a like binding event proximate to the second transistor, is sensed between the gate electrodes of the first and second transistors.

This application is a 371 filing of PCT/US97/05660, filed Apr. 4, 1997;which is a Continuation of U.S. application Ser. No. 08/634,102; filedApr. 17, 1996; and now abandoned.

FIELD OF THE INVENTION

The present invention relates to molecular detection devices.

BACKGROUND OF THE INVENTION

Recently, an increased effort has been directed toward the developmentof chips for molecular detection. In general, a molecular detection chipincludes a substrate on which an array of binding sites is arranged.Each binding site (or hybridization site) has a respective molecularreceptor which binds or hybridizes with a molecule having apredetermined structure. A sample solution is applied to the moleculardetection chip, and molecules in the sample bind or hybridize at one ormore of the binding sites. The particular binding sites at whichhybridization occurs are detected, and one or more molecular structureswithin the sample are subsequently deduced.

Of great interest are molecular detection chips for gene sequencing.These chips, often referred to as DNA chips, utilize an array ofselective binding sites each having respective single-stranded DNAprobes. A sample of single-stranded DNA fragments, referred to as targetDNA, is applied to the DNA chip. The DNA fragments attach to one or moreof the DNA probes by a hybridization process. By detecting which DNAprobes have a DNA fragment hybridized thereto, a sequence of nucleotidebases within the DNA fragment can be determined.

To hasten the hybridization process, a local concentration of target DNAcan be increased at predetermined sites using electric fieldenhancements. Here, each site has an electrode associated therewith forselectively generating an electric field thereby. The electric field isgenerated by applying an electric potential between an electrode at thesite and a counter electrode at a peripheral portion of the chip. Toattract DNA fragments to the site, the polarity of the electricpotential is selected to generate an electric field having a polarityopposite to the charge of the DNA fragments. To de-hybridize the site,an electric field having the same polarity as the DNA fragments can begenerated to repel the DNA fragments from the site.

Various approaches have been utilized to detect a hybridization event ata binding site. In one approach, a radioactive marker is attached toeach of a plurality of molecules in the sample. The binding of amolecule to a molecular receptor is then detectable by detecting theradioactive marker.

Other approaches for detection utilize fluorescent labels, such asfluorophores which selectively illuminate when hybridization occurs.These fluorophores are illuminated by a pump light source external tothe substrate. An external charge-coupled device (CCD) camera isutilized to detect fluorescence from the illuminated fluorophores.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention is pointed out with particularity in the appended claims.However, other features of the invention will become more apparent andthe invention will be best understood by referring to the followingdetailed description in conjunction with the accompanying drawings inwhich:

FIG. 1 is a block diagram of an embodiment of a molecular detectionapparatus in accordance with the present invention;

FIG. 2 is a flow chart of an embodiment of a method of sensing a bindingof a molecule to a molecular receptor at a binding site in a moleculardetection apparatus;

FIG. 3 is a flow chart of an embodiment of a method of sensing amodified electrical characteristic of the transistor;

FIG. 4 is a flow chart of another embodiment of a method of sensing amodified electrical characteristic of the transistor;

FIG. 5 is a flow chart of yet another embodiment of a method of sensinga modified electrical characteristic of the transistor;

FIG. 6 schematically illustrates a differential pair formed by a firsttransistor and a second transistor;

FIG. 7 is a cross-sectional view of another embodiment of an apparatusfor sensing a binding of a molecule at a binding site in a moleculardetection apparatus; and

FIGS. 8 and 9 illustrate a top view and a side view, respectively, of anembodiment of an integrated molecular detection apparatus in accordancewith the present invention.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

Embodiments of the present invention advantageously provide a moleculardetection apparatus which detects the binding or hybridization of amolecule to a molecular receptor by sensing a charge associated with themolecule. A preferred embodiment utilizes a thin-film transistorintegrated with a substrate to define a binding site. The thin-filmtransistor is utilized both to detect binding events and to controlhybridization and de-hybridization. The sensitivity of detection can beenhanced by forming a differential pair using the transistor and asecond transistor at an unhybridized site.

FIG. 1 is a block diagram of an embodiment of a molecular detectionapparatus 10 in accordance with the present invention. The moleculardetection apparatus 10 includes a substrate 12 which supports a bindingsite 14 for receiving a molecular receptor 16. In general, the molecularreceptor 16 is selected in dependence upon a type of molecule which isto be detected. The molecular receptor 16 typically includes abiological or synthetic molecule that has a specific affinity to themolecule to be detected. The molecular receptor 16 can include a chainof at least one nucleotide which hybridizes with a complementary chainof at least one nucleotide included in the molecule. Here, for example,the molecular receptor 16 can include a DNA probe for detecting acorresponding, complementary DNA sequence in the molecule. It is noted,however, that the scope of the invention is not limited to sensing thehybridization of DNA molecules. For example, embodiments of the presentinvention can be utilized to detect RNA hybridization andantibody-antigen binding events.

The molecular detection apparatus 10 further includes a transistor 18integrated or fabricated in the substrate 12. The transistor 18 has agate electrode 20, a source electrode 22, and a drain electrode 24. Asemiconductive channel layer 26 in the transistor 18 electricallycouples the source electrode 22 to the drain electrode 24. Thesemiconductive channel layer 26 is located proximate to the binding site14 so that a conductance between the source electrode 22 and the drainelectrode 24 is modified by a charge associated with a molecule 28 whenthe molecule 28 binds with the molecular receptor 16. The binding of themolecule 28 to the molecular receptor 16 is sensed by sensing a modifiedelectrical characteristic of the transistor 18 which results from thecharge associated with the molecule being proximate to thesemiconductive channel layer 26.

The charge associated with the molecule 28 can be inherent in themolecule 28, such as the inherent charge in a DNA molecule. The chargeassociated with the molecule 28 may also result from a charged memberattached to the molecule 28. For example, the charge associated with themolecule 28 can result from a charged bead being attached to themolecule 28.

Various known technologies can be utilized to form the transistor 18. Ina preferred embodiment, the transistor 18 is a thin-film transistor(TFT). Using thin-film technology, the semiconductive channel layer 26can be formed of an organic material which allows the molecular receptor16 to be bound directly to a surface of the semiconductive channel layer26. Alternatively, the semiconductive channel layer 26 can be formed ofsilicon (such as a-Si or poly-Si), in which case an insulation layer 29can be disposed between the molecular receptor 16 and a surface of thesemiconductive channel layer 26 to provide appropriate passivation. Theinsulation layer 29 can be in the form of a surface oxide layer.

To enhance the hybridization process, the apparatus can include anattachment layer on which the molecular receptor 16 is bound. Theattachment layer is disposed between the molecular receptor 16 and thesurface of either the semiconductive channel layer 26 or the insulationlayer 29.

FIG. 2 is a flow chart of an embodiment of a method of sensing a bindingof a molecule to a molecular receptor at a binding site in a moleculardetection apparatus. As indicated by block 30, the method includes astep of providing a transistor having a semiconductive channel layerlocated proximate to the molecular receptor so that a conductancebetween a source electrode and a drain electrode is modified by a chargeassociated with the molecule when the molecule hybridizes with themolecular receptor. This step can be performed by utilizing anembodiment of a molecular detection apparatus as described herein.

As indicated by block 32, the method includes a step of sensing amodified electrical characteristic of the transistor which results fromthe charge associated with the molecule being proximate to thesemiconductive channel layer upon binding. This step of sensing themodified electrical characteristic can be performed in a variety ofways, three of which being described below.

FIG. 3 is a flow chart of an embodiment of a method of sensing amodified electrical characteristic of the transistor. As indicated byblock 40, the method includes a step of biasing the transistor in apredetermined manner prior to the binding of the molecule with themolecular receptor. Here, a respective, predetermined voltage level isapplied to each of the gate electrode, the drain electrode, and thesource electrode of the transistor.

As indicated by block 42, a step of measuring a first channel currentbetween the drain electrode and the source electrode is performed priorto the binding of the molecule with the molecular receptor. The firstchannel current results from the biasing of the transistor performed inthe previous step.

After measuring the first channel current, the molecule is allowed tohybridize or bind with the molecular receptor. As indicated by block 44,the binding can be field-enhanced by performing a step of applying afirst voltage to at least one of the gate electrode, the sourceelectrode, and the drain electrode. The first voltage is selected togenerate an electric field which attracts the molecule to the bindingsite.

After hybridization, an optional step of dehybridizing any unwantedmolecules from the binding site can be performed. Specifically, asindicated by block 46, a step of dehybridization can be performed byapplying a second voltage to at least one of the gate electrode, thesource electrode, and the drain electrode. The second voltage isselected to provide an electric field which repels unwanted moleculesfrom the binding site. The unwanted molecules can includepartially-bound molecules, for example.

As indicated by block 48, a step of re-biasing the transistor isperformed. Here, the transistor is biased in the same predeterminedmanner as in the step indicated by block 40.

As indicated by block 50, a step of measuring a second channel currentbetween the drain electrode and the source electrode is performed afterthe binding of the molecule with the molecular receptor. The secondchannel current results from the biasing of the transistor performed inthe previous step. Preferably, the first channel current and the secondchannel current are measured for a fixed voltage applied to the gateelectrode.

The modified electrical characteristic is sensed by a step of detectinga difference between the first channel current and the second channelcurrent, indicated by block 52. For example, the modified electricalcharacteristic may be determined when a difference between the firstchannel current and the second channel current is beyond a predeterminedthreshold.

FIG. 4 is a flow chart of another embodiment of a method of sensing amodified electrical characteristic of the transistor. As indicated byblock 60, the method includes a step of biasing the transistor in apredetermined manner. Here, a respective, predetermined voltage level isapplied to each of the drain electrode and the source electrode of thetransistor.

As indicated by block 62, a step of determining a voltage for the gateelectrode to produce a predetermined channel current is performed. Inone embodiment, the predetermined channel current is selected to be nearzero. Here, the voltage applied to the gate electrode is varied todetermine a threshold voltage which nulls out the channel current. Thethreshold voltage which nulls the channel current is proportional to theamount of charge incorporated into the channel layer by the binding. Itis noted that the predetermined channel current need not be near zero inalternative embodiments.

The modified electrical characteristic is sensed by a step, indicated byblock 64, of detecting a difference between a predetermined voltagelevel and the voltage determined in the above-described step. Thepredetermined voltage level can be, for example, a voltage whichproduces the predetermined channel current before hybridization. Hence,the modified electrical characteristic may be determined when the gatevoltage (post-hybridization) which produces the predetermined channelcurrent is beyond a predetermined threshold.

FIG. 5 is a flow chart of yet another embodiment of a method of sensinga modified electrical characteristic of the transistor. As indicated byblock 70, the method includes a step of providing a second transistorwhich is substantially similar to the transistor at the binding site.The second transistor, however, is located at an unhybridized site onthe molecular detection apparatus. The second transistor is electricallyconnected with the transistor to form a differential pair. As indicatedby block 71, a step of detecting a signal, produced by the differentialpair, indicative of a binding of the molecule at the binding site isperformed.

FIG. 6 schematically illustrates a differential pair 72 formed by afirst transistor 73 and a second transistor 74. The first transistor 73is located at a binding site while the second transistor 74 is locatedat an unhybridized site. Physically, the first transistor 73 and thesecond transistor 74 can be located adjacent one another on a substrate.The differential pair is formed by coupling a source electrode 75 of thefirst transistor 73 to a source electrode 76 of the second transistor74.

A binding event can be detected by applying a common voltage to gateelectrodes 77 and 78, and detecting a difference in channel currentsbetween the first transistor 73 and the second transistor 74.Alternatively, the binding event can be detected by detecting a non-zerooffset voltage between the gate electrodes 77 and 78 which producesequal channel currents for the first transistor 73 and the secondtransistor 74.

FIG. 7 is a cross-sectional view of another embodiment of an apparatusfor sensing a binding of a molecule at a binding site in a moleculardetection apparatus. This embodiment utilizes a thin-film transistor 80formed on a substrate 82. Disposed on a top surface of the substrate 82are a gate electrode 84 and an insulation layer 86. A source electrode88, a drain electrode 90, and a channel layer 92 are formed on a topsurface of the insulation layer 86.

A molecular receptor, such as a single-stranded DNA molecule 94, islocated in proximity to the channel layer 92. As illustrated, thesingle-stranded DNA molecule 94 can be attached directly to a surface ofthe channel layer 92. As described earlier, the channel layer 92 can beformed of an organic material which allows the single-stranded DNAmolecule 94 to be directly attached to the surface. Here, the organicmaterial is selected to be compatible with the DNA species and tooptimize the attachment of DNA fragments to the surface.

By burying the gate electrode 84, the source electrode 88, and the drainelectrode 90 beneath the channel layer 92, difficulties associated withpotential-induced denaturation at the electrodes are prevented.

FIGS. 8 and 9 illustrate a top view and a side view, respectively, of anembodiment of an integrated molecular detection apparatus in accordancewith the present invention. The integrated molecular detection apparatusincludes an array of thin-film transistors 100 fabricated on a topsurface of a substrate 102. The thin-film transistors 100 can be formedin a manner similar to that used to construct active matrix displays.

Each of the thin-film transistors 100 is located proximate to arespective one of plurality of binding sites 104. Specific DNA probesare deposited onto each of the thin-film transistors 100. The DNA probescan be deposited using conventional robotic dispensing techniques, orcan be bound specifically into a channel of the thin-film transistors100 using binding techniques known in the art.

In operation as a sequencer or a diagnostic tool, DNA sequences in asample analyte hybridize onto selective ones of the binding sites 104.Field-assisted or thermally-assisted hybridization techniques can beutilized to enhance the hybridization process. After hybridization,unwanted sequences with only partial binding can be dehybridized usingfield enhancement by switching appropriate biases onto at least oneelectrode of the thin-film transistors 100. Alternatively, thermaldesorption can be utilized to dehybridize unwanted sequences.

Thereafter, each of the thin-film transistors 100 is biased fortransistor operation. As described earlier, a gate voltage for each ofthe thin-film transistors 100 can be varied to null out a respectivechannel current. The gate voltage required to null out the respectivechannel current is proportional to an amount of charge incorporated inthe thin-film transistor. The value of the gate voltage can be read-outthrough the active matrix. As previously described, alternativeapproaches to detecting binding events include, but are not limited to,detecting a variation in channel current (measured before and afterhybridization) for a fixed gate voltage, and detecting a signal producedby a differential pair of thin-film transistors.

Thus, there has been described herein a concept, as well as severalembodiments including preferred embodiments of a transistor-basedmolecular detection apparatus and method.

Because the various embodiments of the present invention detect abinding event by sensing a charge associated with a target molecule,they provide a significant improvement in that a transistor integratedin the molecular detection apparatus can be utilized to electronicallydetect the target molecule. To improve detection, the charge associatedwith the target molecule can be enhanced by attaching a charged bead tothe target molecule.

Additionally, the various embodiments of the present invention asherein-described utilize electrodes in the transistor to performfield-assisted hybridization and dehybridization.

It will be apparent to those skilled in the art that the disclosedinvention may be modified in numerous ways and may assume manyembodiments other than the preferred form specifically set out anddescribed above.

Accordingly, it is intended by the appended claims to cover allmodifications of the invention which fall within the true spirit andscope of the invention.

What is claimed is:
 1. A molecular detection method comprising the stepsof: providing a substrate; providing a first transistor integrated withthe substrate, the first transistor having a gate electrode, a sourceelectrode, a drain electrode and a semiconductive channel whichelectrically couples the source electrode with the drain electrode, thesemiconductive channel formed of an organic material; providing amolecular receptor bound directly to a surface of the semiconductivechannel of the first transistor; providing a second transistorintegrated with the substrate and adjacent the first transistor, thesecond transistor having a gate electrode, a source electrode, a drainelectrode and a semiconductive channel which electrically couples thesource electrode with the drain electrode, the semiconductive channelformed of an organic material, the source electrode electrically coupledto the source electrode of the first transistor; and sensing a non-zerooffset voltage between the gate electrode of the first transistor andthe gate electrode of the second transistor which produces equal channelcurrents in the semiconductive channels of the first transistor and thesecond transistor after a molecule has bound with the molecular receptorwithout a like binding event proximate to the second transistor.
 2. Themolecular detection method of claim 1 wherein the molecular receptorcomprises a chain of a plurality of nucleotides.
 3. The moleculardetection method of claim 1 wherein the molecular receptor is receptiveto a chain of a plurality of nucleotides.
 4. The molecular detectionmethod of claim 1 wherein the molecular receptor is receptive to a DNAmolecule.